An Analysis of the Sewage Treatment Processes in Urban Sewage Treatment Plants

With the acceleration of urbanization, the urban population continues to grow, and the volume of urban sewage discharge shows an increasing trend year by year. The large amount of sewage discharge puts tremendous pressure on the urban environment, and the harmful components carried in the sewage, such as various organic substances, microorganisms, and heavy metals, can severely harm both the water and ecological environment. Therefore, urban sewage treatment plants have become important facilities for ensuring water source safety and protecting the ecological environment. This article will explore the sewage treatment processes in urban sewage treatment plants, analyzing their principles, characteristics, advantages, and disadvantages.

1. Components and Pollutants in Urban Sewage

Urban sewage has a complex composition, including organic substances, inorganic substances, microorganisms, and nutrients. Among them, organic pollutants are the primary component of urban sewage, including phenols, acids, hydrocarbons, fats, etc. The presence of organic substances can lead to the rapid growth and reproduction of microorganisms, resulting in the generation of low-molecular organic matter and large amounts of sludge. In addition, urban sewage also contains a significant amount of nutrients, such as nitrogen and phosphorus, which can affect the survival and reproduction of aquatic life.

2. Urban Sewage Treatment Processes

2.1 Pre-treatment

Pre-treatment is the first and crucial step in sewage treatment. It primarily involves processes such as screen grids, grit chambers, and sand sedimentation tanks, with the main function being the filtration of large debris and pollutants. The screen grid removes large objects such as leaves, paper, glass, fabric, and other debris from the sewage. Additionally, there are some difficult-to-remove pollutants in urban sewage, such as sand and sludge, which need to be removed through sand sedimentation tanks.

2.2 Primary Treatment

Primary treatment mainly uses rigid biochemical treatment processes, including regulating tanks, grit tanks, and organic sludge tanks. In this step, pollutants like suspended solids (SS), chemical oxygen demand (COD), and biochemical oxygen demand (BOD) are removed through methods such as sedimentation and particle settling. The selection of tank types depends on the water quality and pollutant concentration. In urban sewage treatment plants, tank shapes are generally rectangular or circular, which have a wide range of applications. The primary treatment process is characterized by simple operation, stable performance, but relatively low removal efficiency.

2.3 Secondary Treatment

Secondary treatment mainly uses the activated sludge method. After sedimentation, aerobic activated sludge is added to the activated sludge tank to further remove substances like nitrogen and phosphorus from the sewage. In the activated sludge tank, aerobic bacteria combine with pollutants to remove them. The required number of tanks for the aerobic, anoxic, and anaerobic zones in the activated sludge tank is larger than that in the primary treatment. While the treatment effect is better, the process is more complex, and its applicability is limited.

2.4 Tertiary Treatment

Tertiary treatment is used for removing high concentrations of heavy metal copper ions. This process utilizes the bio-chelation between microorganisms in the sludge and copper ions to remove copper. However, this method also has the drawback of being difficult to operate and maintain.

3. Conclusion

The sewage treatment process in urban sewage treatment plants is constantly evolving, with various new technologies emerging. However, challenges such as the treatment and removal of toxic and hard-to-degrade substances, handling sewage in low-temperature seasons, and quantifying sludge removal still need to be addressed. Urban sewage treatment will be an ongoing process, requiring collective efforts to build a better, cleaner, and smarter city.